Many load circuits have been employed each of which is configured in a manner that an electronic switch such as a MOSFET is provided between a DC power supply and a load, and the MOSFET is subjected to the PWM control to adjust an electric power supplied to the load to thereby control the output of the load (see a patent document 1, for example).
FIG. 4 is a circuit diagram showing the configuration of a load circuit for driving a motor M101, used for a radiator fan etc. to be mounted on a vehicle, by the PWM control. An electronic switch T101 such as a MOSFET is provided between the motor M101 and the positive electrode terminal of a DC power supply VB (for example, a battery to be mounted on the vehicle). The inductance of the motor M101 is depicted as LM and an armature resistance thereof is depicted as Ra.
The gate of the electronic switch T101 is connected to a driver 102 via a resistor R101. A charge pump 101 is connected to the driver 102. When the driver 102 is supplied with an input signal having a predetermined on/off cycle, the driver 102 outputs the output voltage of the charge pump 101 to the gate of the electronic switch T101 to thereby drive the electronic switch T101 with a predetermined duty ratio according to the PWM control.
As shown in FIG. 4, supposing that a current flowing through the electronic switch T101 is ID and a current flowing through the motor M101 is IM, the current ID becomes equal to the current IM in the case where the electronic switch T101 is turned on to flow a current through a path from the positive electrode terminal of the DC power supply VB to the negative electrode terminal of the DC power supply VB via the electronic switch T101, the motor M101 and the ground. In this case, the electromagnetic energy of LM*IM2/2 is accumulated in the inductance LM of the motor M101.
When the electronic switch T101 is turned off, the current ID becomes 0. However, the current IM tends to continuously flow due to the inductance LM of the motor M101. In order to flow this current into the motor M101 in a circulation manner, a flywheel diode D101 (hereinafter simply referred a “diode”) is provided in parallel to the motor M101. Supposing that a connection point between the electronic switch T101 and the motor M101 is a, the cathode of the diode D101 is connected to the point a and the anode of the diode D101 is grounded.
According to this configuration, the current IM having been flowing through the motor M101 during the on-state of the electronic switch T101 starts to flow in a circulation manner through a path from the motor M101 to the motor M101 via the ground, the diode D101 and the point a when the electronic switch T101 is turned off. Thus, the electromagnetic energy having been accumulated in the inductance LM is converted into the driving torque of the motor M101. In this case, supposing that the current flowing through the diode D101 is a circulation current IF, the circulation current IF becomes equal to the current IM.
The motor current IM (=IF) flows through the armature resistor Ra and the diode D101 to thereby cause a power loss.
Since the power loss at the armature resistor Ra causes the generation of the motor driving torque, this power loss has an effects of suppressing the reduction of the rotation speed of the motor M101 during the off period of the electronic switch T101. On the other hand, a power loss generated in the diode D101 is represented by VF* IF supposing that the voltage drop in the forward direction of the diode D101 is VF. Since this power loss is converted into heat to thereby merely increase the temperature of the diode D101, this power loss does not contribute to the maintaining of the rotation speed of the motor M101.
When the electromagnetic energy accumulated in the inductance LM of the motor M101 disappears as the power loss of the armature resistor Ra and the diode D101, the voltage at the point a increases to the generation voltage due to the inertial force of the motor armature. In this case, the diode D101 prevents a current from flowing toward the ground from the point a.
That is, a part of the electromagnetic energy accumulated in the inductance LM is converted into the torque of the motor M101 and the remaining of the electromagnetic energy is converted into heat generated by the diode D101. Thus, if it becomes possible to reduce the power loss of the diode D101, the conversion ratio into the torque of the motor M101 can be increased, whereby the energy can be utilized effectively and an amount of heat generated from the diode D101 can be reduced.